Spectroscopic ellipsometry is used to investigate the subsurface reactions related to hydrogen diffusion during the annealing at temperatures ranging from 250 degreesC to 650 degreesC of very thin a-Si:H films in stacked structures. A gradual and partial crystallization of the a-Si film develops from both the substrate and the top surface layers depending on annealing time. The SE approach is also applied to the characterization and optimization of the annealing process of ITO/a-Si:H/c-Si heterostructures for solar cells application. The effects of the annealing process on the a-Si:H microstructural modification and on the optical properties of the indium tin oxide (ITO) layer are deduced.